Increasing efficiency plays a central role on the road to generating energy while preserving resources and safeguarding the environment. For example, by 2025 it is aimed to increase the overall efficiency of combined-cycle power plants (a combination of gas turbine power plant and steam turbine power plant) to over 60%. To enable this objective to be achieved, it is necessary to increase the efficiency of gas turbines.
An important parameter in increasing the efficiency of gas turbines is the turbine inlet temperature. The current efficiency of a gas turbine of approx. 38% can be achieved with a turbine inlet temperature of 1230° C. (ISO). Increasing the efficiency of the gas turbine would require an increase in the turbine inlet temperature. For example, in order to increase the efficiency of a gas turbine to 45%, it would be necessary to increase the turbine inlet temperature to approx. 1350° C.
In addition to the use of improved base materials and effective cooling methods, the use of ceramic thermal barrier coatings constitutes a key technology for achieving this objective. On account of the thermally insulating action of the ceramic thermal barrier coating, the surface of the coated component can be exposed to a temperature which is several 100° C higher than without the thermal barrier coating, without the cooling conditions for the component having to be altered. A precondition for the efficient use of ceramic thermal barrier coatings (referred to below as TBCs) in gas turbines, in addition to an inexpensive process technology, is in particular the structural stability and therefore the reliability of the TBC under the conditions of use in gas turbines. For example, for gas turbines used in power plant applications, faultless functioning for over 20000 equivalent operating hours, as they are known, or more has to be ensured. Premature failure of the TBC would consequently lead to the base material of the coated turbine components overheating and possibly to catastrophic damage to the turbine. The operating loss and maintenance costs caused by turbine damage may be considerable and would ultimately cancel out the technological benefit of the TBC.
In future generations of high-efficiency gas turbines, the stresses will move closer and closer to the limits of the TBC's abilities. To ensure that the risk of failure is not uncontrollably increased as a result, the stress parameters which are critical for failure of the TBC have to be measured and checked in operation. Important stressing parameters in this context are the surface temperature of the TBC at critical component positions, known as hot spots, and the time dependency of the surface temperature. The latter is important in particular for the transitions between different operating states of the gas turbine, for example when running up the gas turbine.
Therefore, there is a demand for a suitable sensor element which is suitable for measuring temperature and heat flux in the TBC under operating conditions of a gas turbine.
Pyrometers and thermal imaging cameras are nowadays already in use for measuring the surface temperature of guide vanes in gas turbines. In this case, the radiation emitted by the TBC surface in a defined wavelength region, for example in the infrared or near infrared, is detected and, taking account of the emissivity and the detector sensitivity, converted into an equivalent surface temperature of the TBC. In this context, the generally inadequate knowledge of the wavelength-dependent emissivity of the TBC surface, which may be sensitively influenced inter alia by deposits on the TBC surface (for example rust), presents difficulties.
Another way of measuring surface temperatures is to use thermographic phosphors. A thermal barrier coating with embedded thermoluminescence indicator material and methods for determining the temperature of the thermal barrier coating are described, for example, in EP 1 105 550 B1. To determine the temperature of the thermal barrier coating, the indicator material is excited to fluoresce by means of a pulsed laser. After the excitation pulse has been switched off, the intensity of the fluorescence spectrum drops exponentially with a characteristic time constant t. For example, terbium-doped yttrium aluminum garnet (YAG:Tb) has a monotone decrease in the characteristic time constant t between 700 and 1000° C. The temperature of the indicator material and therefore of the thermal barrier coating in which it is embedded can be ascertained by measuring the time constant, provided appropriate calibration has been carried out. Under certain circumstances, different lines of the emission spectrum may have different decay constants, which may also have different temperature dependencies.
Instead of the time decay properties of the emission intensity of the indicator material, it is also possible to use the intensity ratio of two emission wavelengths to determine the temperature of the indicator material and therefore the temperature of the thermal barrier coating. The intensity ratio is approximately linearly dependent on the temperature of the indicator material, i.e. on the temperature of the thermal barrier coating in which the indicator material is embedded. The measuring of the temperature by means of the intensity ratio is likewise described in EP 1 105 550 B1.
The advantage of using measurement methods which are based on thermographic phosphors is that they are independent of the emissivity of the TBC, which is generally only inadequately known, and of the influence of surface contamination, which often influences the property of the TBC as a heat radiator. The specific properties of the emission spectrum of the phosphor, by contrast, are only slightly influenced by the emissivity and surface contamination.
The restriction on the thermal sensitivity of the phosphors to in each case a specific temperature range and the limited long-term stability of a thermographic phosphor under the thermal and atmospheric conditions of a gas turbine impose limits on temperature measurement by means of emission spectra of thermographic phosphors.